Additional borrowing question

PurpleFairy26

We are coming up to the end of a fixed term deal and would like to fix again. We would also like to take out some additional borrowing for an extension which would add significant value to the property. I called our lender and they said we can do it all in one go as 2 mortgages (and that would be on the total Ltv of both mortgages combined) or I can fix the amount of our current mortgage now at whatever ltv that is and then come back later and get the rest and just that bit will be on a higher ltv so slightly worse rate. Am I missing something, other than risking not getting the additional capital if they won't lend one part 2, surely it would save us some cash getting a chunk of it fixed on a lower rate? Or Can I not do this if we know we want to take out more borrowing?

Any advice greatly received.